This application is based on Japanese Patent Application HEI 11-245220, filed on Aug. 31, 1999, the entire contents of which are incorporated herein by reference.
a) Field of the Invention
The present invention relates to ZnO crystal, its growth method, and semiconductor device using ZnO crystals.
b) Description of the Related Art
ZnO can be grown directly on a sapphire substrate by using, for example, RS-MBE (radical sourcexe2x80x94molecular beam epitaxy). As a Zn source, a Zn solid source in K cell (Knudsen cell) is used. As an O source, oxygen radicals are used which are generated from an oxygen gas source by using RF (radio frequency), ECR (electron cyclotron resonance) or the like.
RF-MBE uses a high frequency of 13.56 MHz most popularly used in commercial systems. O radicals are generated by introducing O2 source gas into an electrodeless discharge tube in an MBE chamber. O radicals are jetted out into the MBE chamber, forming a O radical beam. A ZnO thin film can be grown by radiating an O radical beam and a Zn beam from a K cell to a sapphire substrate at the same time.
Semiconductor devices such as LED (light emitting diodes) and LD (laser diodes) having a p-n junction can be manufactured by using ZnO which is one of group II-V compound semiconductor.
Crystallinity of semiconductor crystal material of semiconductor devices such as LED and LD greatly influences the electric characteristics, optical characteristics and reliability (life time) of semiconductor devices. The better the crystallinity of semiconductor crystal material of a semiconductor device, the better the electric characteristics, optical characteristics and reliability of semiconductor devices.
Conventional ZnO crystal grown directly on a sapphire substrate by RS-MBE is associated with the following problems. A difference of lattice constant between a sapphire substrate and ZnO (a lattice constant mismatch is about 18%) large. A large difference of thermal expansion coefficient therebetween is also as large as about 2.6 times.
Many crystal defects are introduced into grown ZnO crystal.
FIG. 7 shows XRC (X-ray rocking curve) measurement results of ZnO (0002) crystals directly grown on a sapphire (0001) substrate. ZnO crystal was grown by RS-MBE under the conditions of a growth temperature Tg of 650xc2x0 C., a Zn partial pressure Pzn of 1xc3x9710xe2x88x927 Torr, an oxygen flow rate O2 of 2 sccm, and an RF output of 300 W.
A curve of approximately normal distribution having a peak intensity at an omega angle of near 17.5 degrees was observed as seen from XRC measurement results. A full width at half maximum (FWHM) was as large as 0.5xc2x0 (1800 arcsec). It was found from the measurement results shown in FIG. 7 that the crystallinity of ZnO crystal grown under the above-described conditions was not good.
FIG. 8 shows PL (photo luminescence) measurement results of ZnO crystal grown under the above-described conditions. The abscissa represents an energy of PL output light. A sharp peak with a high intensity and always with a narrow FWHM was observed at an energy of near 3.35 eV. The peak energy value of PL is generally coincident with a forbidden band (3.3 eV) of ZnO. This can be considered as a radiation peak to be caused by recombination of electrons and holes between the conductive band and valence band.
A broad peak was also observed in an energy range from about 1.8 eV to about 2.7 eV. This broad peak can be considered as radiation between deep levels existing in the forbidden band. This suggests that ZnO crystal has many crystal defects.
It is an object of the present invention to provide a crystal growth method capable of growing good ZnO crystal on a sapphire substrate while reducing crystal detects in ZnO crystal.
It is another object of the present invention to provide ZnO crystal to be grown on a sapphire substrate, and a semiconductor device using ZnO crystal.
According to one aspect of the present invention, there is provided a ZnO crystal growing method comprising the steps of: growing a low temperature growth ZnO layer on a sapphire substrate at a temperature lower than a single crystal ZnO growth temperature; thermally processing the low temperature growth ZnO layer at a temperature near to a growth temperature of a high temperature growth single crystal ZnO layer higher than the growth temperature of the low temperature growth ZnO layer; and growing a high temperature growth single crystal ZnO layer on the: low temperature growth ZnO layer at a temperature higher than the growth temperature of the low temperature growth ZnO layer.
Single crystal ZnO having good crystallinity can be grown on a sapphire substrate.